Somehow, they just work
by ultravividscene
Summary: A red dress, two controlling step siblings and years worth of pent up, so called hate- and lets not forget frustration. And, of course, it wouldn't be complete without fighting over something.


They were both still panting heavily, laying still on Derek's unmade bed. They made it into their first month of college without seeing each other, without any phone calls or texts, until one night at a frat party. Casey wore a tight red dress that ended mid thigh, and when Derek saw her he suddenly became aware of how tight and uncomfortable his jeans were against his thighs.

He grabbed her by the arm, tugging her away from the scene, ignoring her growls of protest.

"Der-ek!" Casey skirled. She tried to yank her arm out of his grasp but failed. She felt like she was back in high school, fighting with Derek over the TV remote only to find his grasp being too strong for her to win.

Derek ignored her all the way until he reached a dark hallway where the bathrooms were located. He finally let her tug her arm free, the scowl on his tense face ever present. She huffed, glaring up at him with her big blue eyes. Derek took a sly step back, just far enough so that he could see the whole front of her body when he took it all in. He had never seen her in something so sort before, or something so tight, and being her dear step brother, it was his job to make sure she wasn't dressed like a slut... right? Right. The tightness of his pants only seemed to increase. He had no choice but to blame it on the fact that he's a male. That's the only logical explanation. Anything with long legs in a tight dress is meant to turn men on. But does that count for step sisters?

"What-" he said the word like it was dirty, once he finally met her face, "are you wearing?"

Casey blinked, looking down at the red fabric covering her frame. She looked back up at him, eyebrows raised.

"Now, Derek, I know you're a little slow, but I think even you should now that it's called a-"

"I know what you're wearing," he hissed, his fingers automatically tightening around the beer bottle trapped in his hands. "Let me rephrase," he humored her. "Why are you wearing..." as if this was another dirty word, he said, "that..."

It was silent for a moment, except for the music beating through the sound system in the house.

"Because I like it," she finally manged, her expression skeptical.

"I don't," he argued. Casey blinked.

"You say that like it actually matters to me what you think." She was about to turn, but Derek knowing her moves before she makes then, he captured her wrist before she could get away, let alone take a single step.

"It doesn't matter if you care or not. I'm not letting you go back out there dressed like..." here comes that voice again, "... that."

Casey's eyes turned hard and she crossed her eyes. She lifted her chin in attempted to be able to see his face better.

"You don't control me, Derek. No one does."

"Yeah, yeah. Save me your feminist rant and lets skip to the part where you lose and I win and you change out of that dress." This only made Casey more angry.

"Make me." She hissed- oh, he would. Before he could grasp a thought, she was pinned to the wall and his lips were at hers, all teeth and saliva, the total opposite of a Derek Venturi kiss. He awaited a slap, a knee to the hardness that had grown in his pants… but nothing of the sorts came. His lips faltered when he felt her mouth reciprocating. She had kissed him back. That's how it all started.

They ended up pulling away, searching each others eyes for any type of reluctance, but all that was there was lust and need. That's all Derek needed.

No words were said but,  
"Let's get out of here." His voice was low and shaky, and he cringed. Derek Venturi didn't falter for anyone… until Casey Mcdonald, apparently. Did she really have to be the exception to everything?

They drove back to his dorm in silence. Derek couldn't stop himself however, when his hand found her thigh. All he could think was if she didn't want to be touched, she shouldn't have worn such a short dress. He knew his thoughts were wrong, and if he said this to Casey she would slap him and tell him how a women can wear whatever she wants and it doesn't signify that she's looking for sex. He knows that- but Derek will never admit to being wrong, especially not to Casey.

Just like when he kissed her, he was surprised that instead of shoving his hand away or telling him to let her out of the car, she let her legs spread a little further… and fuck, he had never been so turned on in his life. His foot practically burst through the floor of the car when he slammed his foot down into the gas, passing the speed limit by-  
"Der-ek! You're going to get us killed!"

When they got back to his dorm, it was all hands and body, all grind and touch.  
"You're so obnoxious," Casey whined against his mouth, her fingers in his shaggy brown hair tugging with vicious pulls.  
"You're such a fucking princess," Derek practically growled, unaware of the effect that sound had on her. He hadn't expected what happened next to happen, so when it did, he faltered. Casey had shoved him onto his bed, then straddled his legs in one swift move. She stilled. She felt his hardness pressing against the apex of her thighs, and that was enough to provoke a fit of whimpers to flow past her lips. Derek, finding the first of many weak spots, and also not wanting to remain his position at the bottom, gripped her hips and flipped her over onto her back. He shifted one leg between hers, then nudged it under her thigh, angling his hardness against her. She whimpered, clutching her hands to his shoulders immediately following the contact. This wasn't something Derek normally did, but the way her eyes closed and the way her lips parted when she whimpered was enough to keep him going. The layers of clothing against him was driving him mad, but the layers only increased the pressure her used to grind against her.  
"You drive me fukcing crazy, Case." Derek groaned, nipping at her neck as he continued to round his length against her.  
"It wasn't a far drive," her voice wasn't as high pitched as he wanted it to be, and even though the memory of those exact words which he spoke to her five years ago replayed in his mind, he couldn't bring himself to laugh. He would think of it later though, then grin at her wit. But for now, he reached a hand into her underwear, brushing his fingers against her core. He was supposed to be making her feel good, but every touch he gave her and every sound she made somehow connected to his groin.

"Der-ek…" she sighed out. Derek closed his eyes. He just found another way he loved to hear her say his name with two syllables. It wasn't her usual high pitched, annoyed tone. She sounded breathless, confused even, and he knew in that moment that he would do anything he had to that made sure he could hear that sound paired with his name over and over again on an endless loop.

He hitched her legs further apart, reaching for her underwear and tugging them down her long legs. He wanted to appreciate her dance legs, but the ache in his groin prevented that- for the moment.

She helped him with her underwear by lifting her feet backward, then placing them on either side of his own legs.  
"Derek, hurry." She begged, resisting the urge to arch her waist into the air.

Derek unbuckled his belt, the zipper loud in their ears when he pulled it down. He had to reach across her body and tug open a drew from his bedside table. He had a full box of condoms, and Casey wasn't sure if that was a good or bad thing. Either he was having so much sex that he needed a new box, or he had this box for a while, marking it as unused. He was prepared to abandon the condom for a few moments so he could taste her, but suddenly her tiny hand fisted around him, coxing him slowly and- oh god, what was he going to do again?

He has to bite into his lower lip to keep himself from groaning, but then her fist flexes and her wrist jerks and his self control is demolished. He has to resist the urge to fuck into her fist, so he rips open the foil packet and pinches the tip, sighing out a breath thats mixed between relief and pain. He slides it easily onto his length, scrunching his nose upward as the latex clasps around his length. He quickly tosses his shirt over his head. The anticipation bubbling inside of Casey is tangible, matching Derek's perfectly. His hands find her thighs, pushing them a little further, and then her next words have him sinking into her- and oh, fuck, it's even better than he imagined it would be. All those late night imagines and touching himself wishing it was her, when she was on the other side of the wall, probably sound asleep, doesn't compare. He's barely inside of her when he knows, he just know, he's never going to be with anyone other than Casey again. No girl compares- no girl will ever compare. Maybe that says something, and maybe it doesn't. Derek isn't one to over think things, and while this certain person and situation deserves undefined attention and care, right now isn't that moment.

She's tight around him, clenching and it's so good that he pulls out and slams back in, the sound of his thighs against hers bringing on a whole nother feel of ecstasy.  
"Der-ek," her voice is strained, high pitched and needy and he finds his hands pulling down the front of her dress, practically sighing out his appreciation when her breasts move upward, free from any fabric, except for the dress that restricts her breasts from being fully exposed. His one hand finds the back of her thigh, hitching it up as he continues to stroke into her, his other hand holding her hips down. His mouth attaches to a nipple, sucking and pulling and biting and the sounds she makes literally makes his cock throb. Somewhere between increasing thrusts and wet sounds he has trouble stopping the groans that leave his lips, sounding somewhere between "Case," and "fuck," and "so good." But then she clenches around him and heavy moans flow past her lips. He leans down, capturing the sounds with his mouth, and for the first time in all of the times he's had sex, he finishes with the girl beneath him, his throat tight and haling up a groan that floats somewhere in his throat.

He expects it to be awkward, for Casey to cover up and leave, but instead she asks if he has any spare clothes. He's too stunned to move, but when he does he returns to her with one of his black muscle tee's and is fascinated and mesmerized to watching her confidence when she shimmies out of her dress and shrugs on his shirt over…. nothing but flesh.  
"You… don't, mind if I stay, right?" It almost makes him laugh that now she chooses to get shy. Instead of laughing, he slips into his shirt and boxers from before, then strides over to her.  
"Are you hungry?" he asks, backing her toward the kitchen. His hands are touching her waist, and the cut out fabric under her arms from his muscle shirt allows his fingers to make contact with her skin and he's pretty sure keeping his hands off of her, or, harder yet, keeping himself from being inside of her is going to be a real problem now that he knows what she feels like and how she sounds and looks when she comes on his cock. The possibilities are endless… his fingers, his mouth, his thigh...  
"Hungry?" She asks with a widening grin.  
"We can order from somewhere," he suggests. She agrees, but of course they fight over where they're going to order from, and somehow, it just works; because they're Derek and Casey, and somehow, they just work.


End file.
